Electromagnetic interference (EMI) is a class of interference caused by electric and magnetic fields that are generated by many sources such as digital circuits, switching power supplies, electromechanical devices, or even lightning. This interference can be very broadband in nature covering a frequency spectrum from a few kilohertz to many thousands of megahertz.
Radiated EMI is propagated through the air by electromagnetic fields, while conducted EMI is carried via currents on such items as wiring, etc. Radio frequency interference is a subset of EMI generally in the "radio" bands, i.e., above a few megahertz.
EMI comprises the bulk of what is loosely referred to as "electrical noise." Computers and other digital equipment are a source of EMI noise frequencies from around 100 kHz to 10 GHz, and one of the major contributors to radiation is the common mode signal. Designers go to great lengths to rid signals of unwanted noise and to prevent noise from being generated. Only clean, reliable signals must enter and emanate from digital circuits, or else users and nearby users will be plagued with reliability and operability problems.
The Federal Communications Commission (FCC) and other agencies promulgate maximum emission regulations in order to prevent interference with radios, televisions, and other communication devices that depend on broadcast signals. It is an axiom that a circuit that radiates noise is also susceptible to EMI interference as well; thus, noise is sometimes said to be a two-way street. Filters which are designed to reduce EMI susceptibility tend also to reduce EMI emissions, and vice versa.
For relatively low frequency signals, such as frequencies below one megacycle, data lines are often single ended and are relatively easy to filter. At higher data transfer frequencies, filtering is more difficult. For balanced data I/O signals which utilize difference mode signals to carry the information, common mode noise represents a serious problem. At present, several manufacturers offer "three-lead" low pass EMI filters to filter unwanted EMI from data I/O lines. On balanced lines, two such filters would be used.
These three-lead filters typically comprise two ferrite beads inserted in series with an I/O line, with a capacitor to ground from the midpoint of the two series-connected ferrite beads. To date, however, these three-lead filters have been unable to accomplish the required task: remove unwanted common mode noise while leaving the difference mode signal undisturbed. The three-lead low-pass approach has been shown to be unsatisfactory for balanced I/O lines because the filters themselves introduce an unwanted frequency characteristic into the difference mode signals.
The problem with introducing a frequency characteristic into the difference mode signal is that the data can be affected. For example, if a 10 MHz data signal is present, and a conventional low pass filter is used to attenuate common mode noise above 30 MHz, the low pass filter can disturb the timing of the 10 MHz data signal and cause distortion.
Thus, there has been a great need for an economical common mode filter which removes unwanted common mode signals from balanced high-speed data I/O lines and which leaves difference mode signals undisturbed.